Conventional xenon lamps, in general, consist of the electrodes made of materials such as thorium and tungsten, sealed in both ends of a quartz glass tube, and a xenon gas confined in the tube. The electrodes are connected to an electric circuit to produce light upon discharge of current through the xenon.
The electrodes are heated by the discharge of the current. Especially the tips of the electrodes are heated to a high temperature, and the metals of the electrodes are evaporated, although in very small amounts, and are condensed and adhered onto the entire wall of the tube, giving rise to the phenomenon known as blackening. Furthermore, volatile matter contained in the electrodes is emitted, which is a cause of white turbidity. It is common with such lamps that the quantity of light is reduced by about 10% after 100 hours of burning. Therefore, the conventional xenon lamps are very unsatisfactory as a source of light for use, for example, in light fastness testing machines and various optical measuring instruments which require a stable or constant quantity of light.